Abstract

Nanocrystalline nickel-zinc ferrites Ni0.5Zn0.5Fe2O4 thin films have been synthesized via the electrodeposition-anodization from the aqueous sulfate bath. The electrodeposited (Ni-Zn)Fe2 alloy was anodized in aqueous 1 M KOH solution to form the corresponding hydroxides which annealed at different temperatures ranging from 800 to 1000∘C for various periods from 1 to 4 h, to get the required ferrite. SEM micrograph of the formed ferrite particles, annealed at 1000∘C for 4 h appeared as the octahedral-like structure. A good saturation magnetization of 28.2 emu/g was achieved for Ni0.5Zn0.5Fe2O4 thin film produced after the aforementioned conditions. The kinetic studies of the crystallization of Ni0.5Zn0.5Fe2O4 films appeared to be first-order reaction and the activation energy was found to be 10.5 k Joule/mole.

1. Introduction

In recent years, soft magnetic Ni-Zn ferrite films with good magnetic properties, high resistivity, and low coercivity have critical need for high-frequency applications such as RF broad “band choke band,” planar inductors, magnetic sensors, transformers cores, rod antennas, microwaves devices, and telecommunication [1–6]. Therefore, numerous deposition techniques are used for synthesis of required ferrites thin films. They include ferrite plating [7], chemical vapor deposition [8], sputtering [7], dip coating [9], spray pyrolysis [10], and pulsed laser deposition [11] processes. The main difficulties of these methods are that the substrate during or after deposition must be kept at high temperatures, which imposes restrictions on the selection of the substrate material [7]. Also, some of these processes require expensive equipment for a high degree of control. Furthermore, on sintering, the possibility of nonreproducible products and toxic gases are produced [12]. To avoid the problems arising from the high-temperature processes, room temperature synthesis of metal ferrites thin films using electrochemical deposition methods has attracted much attention [13, 14]. In comparison, the electrodeposition technique provides not only the possibility of using low synthesis temperature but also low cost of starting materials and a high purity of the product yield [15–17].

In our previous work, Ni-Zn ferrites thin films have been synthesized via the electrodeposition from ethylene glycol nonaqueous solution [13]. The results obtained indicate the production of NixZn1-x Fe2O4. A high saturation magnetization (48 emu/g) is achieved for Ni0.5Zn0.5Fe2O4 phase. The activation energy of its crystallization is found to be 32 kJ/mol.

The present study aims to prepare Ni0.5Zn0.5Fe2O4 thin film using the aqueous electrochemical deposition method. The electrochemical properties were examined using galvanostatic and cyclic voltammetry. The changes in the crystal structure, crystallite size, microstructure, and magnetic properties of the produced ferrite films were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and vibrating sample magnetometer (VSM) techniques. Moreover, the reaction kinetics and the activation energy were determined.

2. Experimental

The Electrodeposition of (Ni-Zn)Fe2 alloys were conducted from aqueous bath containing 0.1 M NiSO4, 0.1 M ZnSO4 and 0.1 M (NH4)2Fe(SO4)2. All solutions were prepared, immediately before each experiment by dissolving the requisite amount of analytically grade metal sulfates in de-ionized water. The substrate used was copper as a cathode and pure graphite rod as anode. The bath was stirred at 800 rpm.

Cyclic voltammetric (CV) tests were performed at room temperature using a conventional three-electrode cell, in which copper was used as working electrode, the auxiliary electrode was a graphite rod. The reference electrode used was a saturated mercurous sulfate electrode (MSE), 𝐸=650 mV versus standard hydrogen electrode (SHE). The deposition potentials were determined from the polarization curves. Cyclic voltammograms were performed with a computer-controlled potentiostat (Volta-lab 21).

Electrodeposition was identified galvanostatically using constant currents ranging from 0.25 to 1A. Apparent current densities were obtained by dividing the applied current by macroscopic surface area of the deposit. The deposition conditions were optimized to get good quality of (Ni-Zn) Fe2 alloy films with maximum thickness.

The alloy films were anodized using aqueous 1 M KOH. The anodization current density and time were optimized to get well-adhered oxide films to the substrates. After anodization, the films were washed with distilled water and annealed after drying.

The crystalline phases in the different annealed ferrite samples were investigated using X-ray diffraction (XRD) on a Brucker axis D8 diffractometer using the Cu-Kα (𝜆=1.5406 A°) radiation and secondary monochromator in the range 2θ from 20° to 80°. The ferrite particle morphologies were examined by Scanning Electron Microscope (SEM) (JEOL—model JSM-5400). The magnetic properties of the ferrites were measured at room temperature using a vibrating sample magnetometer VSM (9600-1 LDJ, USA) in a maximum applied field of 15 kOe. From the obtained hysteresis loops, the saturation magnetization 𝑀𝑠, remanence magnetization ratio 𝑀𝑟, and coercivity 𝐻𝑐 were determined.

3. Results and Discussion

3.1. Cyclic Voltammetry (CV)

Figures 1(a)–1(f) showed the cyclic voltammogram (CV) of 0.1 M of ZnSO4, NiSO4, (NH4)2Fe(SO4)2, Ni-Zn alloy, Ni-Fe alloy, and (Ni-Zn)Fe2 alloy, respectively, from aqueous sulfate bath. Voltammetric studies were performed within the range of 0 to −2 V using a scan rate (S.R.) of 10 mV s−1. Cyclic voltammogram of ZnSO4 (Figure 1(a)) was characterized by one cathodic peak (C) and one anodic peak (A). This cathodic peak appeared at potential around (𝐸=−0.97 V) for nonagitated solution and at −0.48 V when stirred at 1400 rpm (C1). This cathodic peak represents the reduction of zinc (II) ions to metallic zinc (1). In previous studies, the reduction process associated with this peak is controlled by mass transfer mechanism [18]Zn+2+2𝑒−⟷Zn(1)
On reversing the scan in positive direction at −2 V an anodic peak appeared around −0.7 V for nonstirred solution and −0.5 V for agitated one. This peak is associated with the oxidation of metallic zinc, formed during the scan in negative direction. Moreover, the cathodic current densities were shifted from −5 mA cm−2 for nonagitated solution to, −12.8 mA cm−2 for agitated ones.

In comparison, the CV of 0.1 M of NiSO4 (Figure 1(b)) is characterized by one cathodic peak at potential −0.59 V. For instance, increasing the agitation rate from 0 to 1400 rpm, the cathodic potential shifted from −0.59 to −0.50 V and the current density changed from −6.3 to −18.2 mA cm−2. The increment in the current densities values can be ascribed to the hydrogen evolution reaction.

Figure 1(c) shows the CV of 0.1 M of ammonium ferrous sulfate solution. Iron electrodeposited is observed at potential −0.45 V, and on reversing the scan direction anodic peak appeared at potential −0.68 V for nonstirred solution. However, agitating the solution shifts the current density from −0.7 A cm−2 for nonstirred solution to −2.2 A cm−2 for the agitated solution at 1400 rpm.

Figure 1(d) represents the cyclic voltammogram of Ni-Zn alloy. It is clear that one cathodic peak (C) was formed at (𝐸=−0.89 V) for nonagitated solution. This peak probably represents the codeposition of Ni-Zn alloy. However, this cathodic peak was shifted to −0.58 V on agitating the solution. For instance, agitating the solution increased the current density from −39 to −91 mA cm−2. Two anodic peaks were appeared when the scan is performed in the positive direction, (A1, A2) at (𝐸=−0.850, 𝑉=−0.185 V). The anodic peaks: A1 represents the dissolution of zinc metal, while A2 represents the dissolution of Ni-Zn alloy [19].

Cyclic voltammogram in Figure 1(e) is characterized by one cathodic peak (𝐸=−0.80 V) which is supposed to be for the electrodeposition of Fe-Ni alloy with nonstirred solution and one anodic peak at −0.64 V which is supposed to be for the electrodissolution of Fe-Ni alloy. Increasing the agitation rate from 0 to 800 rpm shifted the current density from −0.77 to −1.27 A cm−2 while the codeposition potential was shifted from −0.80 to −0.53 V. Ni-Zn alloy and Fe-Ni alloy are considered as anomalous alloys [20].

Figure 1(f) represents the cyclic voltammograms for the electrodeposited (Ni-Zn)Fe2 alloy. It is observed that one cathodic peak C (𝐸=−0.58 V) was obtained with nonagitated solution. Agitating the solution shifted the cathodic potential peak to more positive one. For instance, the potential was shifted from −0.58 V for nonagitated solution to −0.57 V for agitated one at 1400 rpm. In addition, agitating the solution changed the solution cathodic current density from −52 to −172 mA cm−2.

3.2. Chronoamperometric Study

Scharifker and Hills suggested model to describe the nucleation process during initial few seconds using chronoamperometric technique. The nucleation process may be either progressive or instantaneous. Progressive nucleation corresponds to slow growth of nuclei on a less number of active sites, all of these sites activated at the same time. Instantaneous nucleation corresponds to fast growth of nuclei on many active sites, all activated during the course of electroreduction [21].

Figure 2 represents the potentiostatic current versus time (I-t) transients for nucleation and growth of (Ni-Zn)Fe2 alloy at different potentials ranging between 570 and 1400 mV. The transients were separated into three regions. The first region corresponds to short times (𝑡<0.5 s). In this region the decrease in the cathodic current density was related to the charging of a double layer. The second region is related to the crystal nucleation process and growth of the crystals formed during the first region. The cathodic current densities of this region achieve its maximum value. The third region corresponds to the decline in the current density, which represents the diffusion process [21].

The transients have been analyzed by comparing the chronoamperometric curves to the dimensionless theoretical curves for the diffusion-controlled nucleation and growth of crystals in three dimensions (3D) proposed by Saba et al. [13].

The expressions for the instantaneous and progressive nucleation are given by following equations, respectively [20],𝑖2𝑖2max𝑡=1.9542max𝑡𝑡1−exp−1.2564𝑡max2,𝑖(2)2𝑖2max𝑡=1.2254max𝑡𝑡1−exp−2.3367𝑡max22,(3)
where 𝑖max and 𝑡max are the maximum current density observed at the maximum time 𝑡max.

The fitting of the experimental curves for the theoretical curves was shown in Figures 3(a) and 3(b).

3.3. Preparation of the (Ni-Zn)Fe2 Alloy

The current efficiency for electrodeposition of (Ni-Zn)Fe2 alloy was studied at different current densities ranging from 52 to 204 mA cm−2 at 800 rpm stirring value for bath including zinc sulfate, nickel sulfate, and ammonium ferrous sulfate in Ni : Zn : Fe molar ratio 0.5 : 0.5 : 2 at room temperature.

According to Sartale et al. [22], the reduction of divalent cations, Ni (II), Zn (II), and Fe(II), is two-step process. the first step one electron takes up to make monovalent adsorbed cations Ni(I)ads, Fe(I)ads and Zn(I)ads and then the second electron takes it to deposited neutral atom. Hydrogen evolution due to reduction of protons was considered as side reaction [13]; side reaction reduced the current efficiency and the kinetics of the metals reduction. The current efficiency of the (Ni-Zn)Fe2 alloy deposition was calculated using the simple relation:𝑚𝜑=100𝑥exp𝑚th,(4)
where 𝑚exp and 𝑚th are the experimental and theoretical masses of the (Ni-Zn)Fe2 alloy. The theoretical mass of (Ni-Zn)Fe2 alloy (𝑚th) can be calculated according to Faraday’s law [23]. The change in the current efficiency with the current density is given in Table 1. The results showed that the current efficiency increases initially with increasing the current density up to 102 mA cm−2 and decreases afterwards within the range studied. About 72.8% current efficiency is obtained at current density of 102 mA cm−2. With further increase in the current density up to 102 mA cm−2, the protons reduction rate is higher than the deposition of metals (Figure 4). Further increase in the current density only leads to increase in the rate of hydrogen reduction which subsequently decreases the current efficiency [23].

Table 1: The effect of current density on current efficiency for the electrodeposited (Ni-Zn) Fe2 alloy.

3.4. Anodization of the Alloy

The anodization of (Ni-Zn)Fe2 alloy thin film from the aqueous solution was performed using 1 M KOH at different annealing temperatures from 800 to 1000°C to form Ni0.5Zn0.5Fe2O4 films.

XRD diffraction patterns in Figure 5 show the produced Ni0.5Zn0.5Fe2O4 films at different annealing temperatures from 800 to 1000°C for 4 h. The results confirmed that the iron oxide α-Fe2O3 (JCPDS no. 73–603) is formed as impurity with the spinel Ni0.5Zn0.5Fe2O4 phase (JCPDS no.08–0234) at 800°C. Peaks at 2θ of 30.07, 35.42, 37.05, 43.09, 63.42, 56.94, 62.56, 71.02, 74.02 related to XRD diffraction planes (220), (311), (222), (400), (422), (511), (440), (620), and (533) of Ni0.5Zn0.5Fe2O4 phase are identified. Single phase of the spinel nickel zinc ferrite films appeared with increasing the annealing temperature from 850 to 1000°C. The crystallite sizes for the most intense peak (311) plane were estimated from XRD data based on Debye-Scherrer formula [13].The crystallite size of the ferrite films is increased with increasing the annealing temperature. Hence, it increased from 27 nm at 800 to 32 nm at 850°C then to 39 nm at 1000°C.

Figure 5: XRD patterns of the produced Ni-Zn ferrite at different annealing temperatures for 4 h.

Figure 6 shows the SEM micrographs of Ni0.5Zn0.5Fe2O4 films annealed at (800, 850, 1000°C) for 4 h. The micrographs of Ni0.5Zn0.5Fe2O4 films in Figures 6(a) and 6(b) displayed spherical particles with a high agglomeration. Increasing annealing temperature to 1000°C octahedral-like structure was formed (Figure 6(c)).

Room-temperature M-H hysteresis loops of Ni0.5Zn0.5Fe2O4 thin film samples annealed at different temperatures are given in Figure 7. The results indicated that the saturation magnetization increased with increasing annealing temperature to 1000°C, as the result of the increasing in the crystallite size, crystallinity, homogenous and clear microstructure. Furthermore, they indicated the decrease of the iron oxide content with increasing the annealing temperature. The 𝐻𝑐 is intensively influenced from magnetic anisotropy including magnetocrystalline and strain-induced anisotropies. Increasing annealing temperature resulted in a decrease in crystal defects and internal strains. As a result, the coercivity was decreased due to a decrease in magnetic anisotropy and increasing the crystallite size [13, 24].

The value of saturation magnetization is lower than the previous obtained value by the authors for Ni0.5Zn0.5Fe2O4 powders (𝑀𝑠=85 emu/g) synthesized via the coprecipitation method [24]. Furthermore, the value is also lower than the value obtained of film using the electrodeposition from nonaqueous medium (𝑀𝑠=48.0 emu/g) using ethylene glycol bath [13]. The results can be attributed on a basis of the decrease in the crystallite size of ferrite films prepared through electrodeposition from the aqueous medium (39.0 nm) than that formed using electrodeposition from non aqueous medium (62.4 nm) or ferrite powders obtained using a coprecipitation technique (81.6 nm).

3.5. Kinetics of Electrodeposition Reaction

XRD patterns of Ni-Zn ferrites thin films annealed at 850°C for various periods from 1 to 4 h are shown in Figure 8. The intensity of the peaks increases with increase of annealing time. This indicates that the crystallinity of the products is improved with the annealing time.

Figure 8: XRD patterns of the produced Ni-Zn ferrites annealed at 850°C for the various periods from 1 to 4 h.

From XRD patterns in Figure 8 the integral intensity of the (311) peak 𝐼𝑥 of the spinel Ni-Zn ferrite phase is calculated by measuring the area under the curve [13].

The intensity of the (311) peak 𝐼𝐴 of the film obtained after annealing for 4 h is used as a standard of crystallization, because of no increase in the intensity of the peak after 4 h. 𝐼𝑥 has intensity of (311) peak at time 𝑡, and the fraction of crystallization in the reactive system can be calculated by the formula of 𝑥=𝐼𝑥/𝐼𝐴. Isothermal crystallization can be described by the Avrami transformation kinetic equation [13] 𝑥=1−exp(−𝑘𝑡𝑛),(5)
where 𝑛 is Avrami exponent (constant), depending on the details of the nucleation and growth mechanisms, 𝑥 is the fraction of crystallization, 𝑘 is reaction rate constant, and 𝑡 is reaction time. For isothermal condition,[]ln−ln(1−𝑥)=ln𝑘+𝑛ln𝑡.(6)
Plotting of ln[−ln(1−𝑥)] as a function of ln𝑡 yields the values of 𝑛 as shown in Figure 9. The straight line obtained showed the mechanism of the crystallization is a random nucleation, and with nucleation as the rate-determining step, could be applied. For this system, the rate of crystallization is controlled by the nucleation in an assemblage of identical reactant fragments and the first-order expression is obeyed in random nucleation, as follows [13]. The temperature dependence of 𝑘 could be described as a simple Arrhenius behavior, that is,
−𝐸𝑘=𝜈exp𝑅𝑇,(7)
where 𝜈 is the frequency factor, 𝐸 is the apparent activation energy of crystallization, and 𝑅 is the gas constant. Taking logarithms of (7)𝐸ln𝑘=ln𝜈−𝑅𝑇(8)
By plotting the ln𝑘 values versus the reciprocal of absolute temperature Arrhenius plot was obtained. From the slope of Figure 10, the activation energy of crystallization was found to be 10.5 kJ/mole which is lower than the values of activation energies reported by Wang and Kang who used the hydrothermal process for synthesis of Ni-Zn ferrites nanoparticles [25] and that obtained by electrochemical deposition process from ethylene glycol [13]. This can be explained by the high dielectric constant of ethylene glycol which was used as solvent in the electrochemical process; however the electrodeposition reaction required much energy to proceed.

Figure 9: Avrami plot of ln[−ln(1−𝑥)] versus ln𝑡 for isothermal crystallization of Ni0.5Zn0.5Fe2O4 films annealed at different temperatures from 800 to 1000°C for different annealed times from 1 to 4 h.

Figure 10: Arrhenius plots of ln𝑘 values against the reciprocal of the temperature for determination the activation energy of Ni0.5Zn0.5Fe2O4 films formed by electrodeposition technique.

4. Conclusions

The results can be summarized as follows.(i)The deposition potential of (Ni-Zn)Fe2 alloy was −0.58 V. For instance, the current density with high current efficiency 72.8% for the electrodeposition of the (Ni-Zn)Fe2 alloy was 102mA cm−2. The mechanism of nucleation and growth of electrodeposited (Ni-Zn)Fe2 alloy was instantaneous mechanism.(ii)Ni0.5Zn0.5Fe2O4 was formed after anodization in 1 M KOH and annealed at 850–1000°C for 4 h.(iii)High saturation magnetization 𝑀𝑠=28.2 emu/g was achieved for Ni0.5Zn0.5Fe2O4 phase annealed at 1000°C for 4 h.(iv)The crystallization of Ni-Zn ferrite films is approached to be first-order reaction and the activation energy was found to be 10.5 kJ/mol.